1. Field of the Invention
This invention relates to the treatment of food, in particular the treatment of dairy products with carbon dioxide.
2. Description of the Related Art
It has been known for decades to use CO.sub.2 as a preservative food products.
There has recently been renewed interest in the storage of raw milk in an atmosphere of CO.sub.2 (see, for example, the article "The Use of Carbon Dioxide for the Preservation of Milk" by J. S. King and L. A. Mabbitt in the Technical Series, Society for Applied Bacteriology, No.22, Preservatives in the Food, Pharmaceutical and Environmental Industries, 1987, pages 35 to 43). Lately, storage of dairy products such as yoghurt, quarg, portion-packed cheese, dessert products and dried milk in an atmosphere of CO.sub.2 only or of CO.sub.2 mixed with other gases has been used commercially to a certain extent. (See the article "Carbon dioxide--Its uses in the Dairy Industry" by H. G. Corinth in Deutsche Molkerei Zeitung 1982, Vol. 103, pages 942-952).
Carbonization of milk with carbonic acid or with CO.sub.2 gas to extend its storage or shelf life is also known. As regards carbonation using carbonic acid, Federal German Patent 961856 is directed to solving the problem of preparing a non-setting, carbonic-acid-containing, long-life, sour milk and this is done by heating fresh milk to an ultra-high temperature of 125.degree. C. to kill all microorganisms in the milk, adding a suitable culture as a 0.5 to 1% addition, carbonating in a sweet state by saturation at +5.degree. C. with carbonic acid to eliminate oxygen, filling into sterilized bottles and sealing, placing the bottles into a room temperated to 20.degree. C., and gradually raising the temperature until, after about 20 hours, so much lactic acid has been formed by the propagation of the lactic acid bacteria that the milk curdles in the bottles. With regard to carbonation using CO.sub.2 gas, as an example the article "CO.sub.2 --Treatment of Milk for Condensation and Drying" by Thomas Eie, Gunnar Rysstad and Helge B. Castberg in Nordeuropaeisk Mejeritidsskrift No. 3-4/87, discloses that the development of the total number of bacteria, psychrotrophic bacteria, coliform bacteria, as well as moulds and yeasts was considerably inhibited in milk which had been nearly saturated with CO.sub.2 (pH below 6.0), when such milk was stored at 5.degree. and 8.degree. C. In another example, the article "Unrefrigerated dahi can keep one month", in Indian Dairyman, Volume 21, No.9, pages 261-262, 1969, discloses that research work showed that bottled dahi of long keeping quality could be prepared by using selected strains of lactic acid bacteria. It also discloses that sweet dahi, sour dahi, acidophilus milk and yoghurt were prepared which could be stored at room temperature and at 30.degree. C. for more than 10 days. It adds that, if the heat treated milk were carbonated prior to inoculation, the non-lactic contaminants were prevented from growing and by such treatment, dahi had been kept for more than a month without refrigeration. However, in its detailed description of the process, in which good quality milk is boiled for 3 to 5 minutes and cooled to about 30.degree.-35.degree. C., 1.5 to 2% of active and pure starter is added and later the milk is transferred to cleaned and sanitized milk bottles which are then tightly closed with sterile caps, the milk is carbonated (by passing CO.sub.2 from a cylinder at a pressure of 60 lbs/sq inch for 1 minute) after adding of the starter.
Indian Patent 140453 discloses a process for preparing yoghurt from milk in which firstly standardization is carried out by addition of skimmed milk powder to standard milk in a pasteurizer and pasteurized at 85.degree. to 90.degree. C. for 30 minutes. The mixture is homogenized at a pressure of 2,500 to 3,000 lbs/sq inch and instantly cooled to 28.degree. C. The mixture is then pumped to an inoculation tank in which 2.5 to 3% of a yoghurt culture is injected. Then the inoculated mixture is carbonated by injecting CO.sub.2 gas. Following this, the carbonated mixture is incubated in the tank at 42.degree. C. for 3 to 4 hours. Then either the incubated mix is pumped to another vessel in which it is maintained at 20.degree. C. for 10 hours, or incubation in the inoculating tank at 42.degree. C. is undertaken for a longer period. Then the coagulum is broken by stirring and the product is packed into containers. Alternatively, for a set type of yoghurt, inoculation occurs at the beginning of the incubation stage and the mix is packed into containers for incubation.
A major microbiological problem during storage of yoghurt is yeast and mould growth on the free surface. In an article "Einfluss der Joghurtbegasung auf das Wachstum von Schimmel und Helen" ("Effect of Gas-flushing of Yoghurt on Mould and Yeast Growth") by Drs. E. Fluckiger and F. Walser in Molkereitechnik 1973, pages 47 to 57, flushing of the headspace in the storage package with CO.sub.2 has been proposed to reduce this problem.
The article "Performance of yoghurt cultures in stored raw and pasteurized milks" in Cultured Dairy Products Journal, Volume 19, No. 1, pages 24, 27, 28 and 33, 1984, discloses changes occurring when raw and pasteurized milks are stored at low temperature and then used for the preparation of yoghurt and also discloses the effects of incorporating whole milk powder to increase total solids content of milk and certain additives on growth and acid production by starter cultures. In relation to the use of pasteurized milks, laboratory pasteurized milk samples stored for varying periods (of from one to five days) at low temperature (5.degree.-7.degree. C.) were withdrawn and used for yoghurt making. One set of pasteurized milk samples was not subjected to any heat treatment and the other set was heated to 90.degree. C. for 10 mins. Both sets were inoculated and then incubated at 44.degree. C. for 4 hours. The results showed that when stored pasteurized milks were inoculated immediately without heat treatment, the starters grew poorly and produced less acid than with fresh pasteurized milk samples. However, in samples subjected to heat treatment, growth and acid production were marginally higher in stored pasteurized milks than in fresh pasteurized milks. The article also discloses the effects of incorporating additives to stored pasteurized milk with a view to avoiding re-heating. One additive employed was sodium formate, which was successful. The other additive employed was CO.sub.2 gas. Stored pasteurized milk samples in flasks fitted with rubber bungs were flushed with CO.sub.2 gas through the budge for varying periods up to 60 secs. After flushing, the milk samples were inoculated with a yoghurt culture and incubated at 44.degree. C. for 4 hours, the bungs being replaced with sterile cotton wool plugs. After flushing, the initial acidity of the milk was increased up to 0.31%; whilst acid production by yoghurt cultures was also increased by CO.sub.2 flushing, an acidity of 0.8% being recorded when CO.sub.2 was flushed for 60 secs. The article concluded that there was increase in total acidity as well as developed acidity in CO.sub.2 flushed samples, indicating that CO.sub.2 atmosphere stimulated the starter activity.
Yoghurt is the coagulated milk product obtained by lactic acid fermentation through the action of Lactobacillus bulgaricus and Streptococcus thermophilus. The organisms are thermophilic and have optimum growth temperature from 40.degree.-45.degree. C.
A conventional process for yoghurt production comprises high-temperature heat treatment of already pasteurized milk, for a time and at temperature-giving a peroxidase negative test of the heat-treated milk. Such high-temperature heat treatment can be a batch treatment at 80.degree.-85.degree. C. for 20-30 minutes or a continuous, on-line treatment at 90.degree.-95.degree. C. for 3 minutes, cooling to an incubation temperature of 43.degree. C., inoculation of a yoghurt starter and incubation for 4-5 hours. Alternatively, an incubation temperature of 30.degree. C. for 12-14 hours can be used. The inoculation can take the form of introducing a starter culture into a fermentation tank through an openable cover or from a starter tank in which the culture is pre-mixed with milk.
The incubation time is critical for development of the right texture and aroma of the yoghurt. Reduction of the incubation time would be of great importance for the dairies, since it would result in less energy consumption, better utilization of incubation tanks and easier planning of production.
Commercial starter laboratories are putting a lot of research into improving the activity of their bacterial starters, both by classical selection and by genetic engineering.